Volumetrics with an alpha background, can it be done in cycles?
You can try using a larger cube or a sphere and lessening the density so that it fades off more slowly.  But the reason it's rendering so slowly is that volumetrics are hard to compute.  What might be better for you is to render it without the volumetrics and instead use the &quot;Glare&quot; node in the compositor (Note, you need to use the &quot;Mix -&gt; Screen&quot; node to get &quot;Glare&quot; to work with transparent backgrounds.  See the example below).